Heretofore, many compounds, each of which has a crystal structure of garnet, have been known. Natural garnet is a silicate mineral, and those with high transparency have been valued as jewelry since ancient times. Then, sand of the garnet is industrially used as abrasive.
As famous garnet, there are almandine (Fe2+3Al2(SiO4)3), grossular (Ca3Al2(SiO4)3), and andradite (Ca3Fe3+2(SiO4)3). Moreover, there are also pyrope (Mg3Al2(SiO4)3), spessartine (Mn3Al2(SiO4)3), uvarovite (Ca3Cr2(SiO4)3), and the like.
Here, a compound represented by Y3Al2(AlO4)3, (which is hereinafter also referred to as YAG), is an artificial mineral synthesized based on the garnet, and is widely known as a name of yttrium aluminum garnet. Then, YAG is used for the purpose of a solid-state laser, translucent ceramics, a phosphor and the like (for example, refer to Non-Patent Literature 1). Moreover, it is known that the YAG includes many modification examples. Typical modification examples of YAG include Tb3Al2(AlO4)3 (for example, refer to Patent Literature 1), Y3Ga2(AlO4)3 (for example, refer to Non-Patent Literature 1), Y3Mg2(AlO4)(SiO4)2 (for example, refer to Patent Literature 2) and the like.
Here, the “phosphor” stands for a compound that emits fluorescence by being given a stimulus such as ultraviolet excitation. Then, when extranuclear electrons of a specific atom that composes the compound are excited by the ultraviolet ray and the like and return to a ground state thereof, then visible light is emitted due to a difference in energy level. For example, rare earth ions and transition metal ions (Ce3+, Tb3+, Eu3+, Mn2+, Mn4+, Fe3+, Cr3+ and the like), each of which functions as a light emission center, are contained in such a compound as YAG, whereby the phosphor is obtained.
Then, a YAG:Ce phosphor activated by Ce3+ is particularly known as a high-efficiency phosphor, and is used in numerous light-emitting devices (for example, refer to Patent Literatures 3 and 4 and Non-Patent Literature 1). The YAG:Ce phosphor is characterized in, at a time of being irradiated with a corpuscular ray or an electromagnetic wave, being excited and emitting super short-afterglow visible light from yellow to green (for example, refer to Non-Patent Literature 1).
Meanwhile, a YAG:Eu phosphor activated by Eu3+ is also known, and study thereof as a red phosphor for a plasma display device (PDP) is made (for example, refer to Non-Patent Literature 1).